It is known that a rhodium salt can be added during the formation of grains of silver halide in order to increase a contrast of a silver halide emulsion. Rhodium salts are very useful in producing silver halide photographic emulsions, because the gradation of the photographic emulsions can be controlled by varying the amount of rhodium salt added.
High contrast silver halide photographic emulsions prepared by the addition of a rhodium salt have a remarkably low sensitivity as compared with emulsions not using a rhodium salt. Also, they have serious defects in that the properties of the resultant emulsions are not stable, and they can become sensitized during storage to change to a low contrast material. For the purpose of overcoming these defects, a process has been known which comprises adding a cadmium salt during production of silver halide photographic emulsions containing a rhodium salt, as described, for example, in U.S. Pat. No. 3,488,709. However, it is not preferred to use light-sensitive materials containing a cadmium salt, because the cadmium salt leaches into processing solutions used for development and fixing, and into a water used for washing, during photographic processing, to cause problems such as harmful influences upon humans and environmental pollution.
Furthermore, a process is known which comprises adding a water-soluble rhodium salt and a nucleic acid decomposition product before the conclusion of the first ripening of a silver halide photographic emulsion, as described in Japanese Patent Application (OPI) No. 23618/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). The addition of a nucleic acid decomposition product to silver halide photographic emulsions is a well known technique, which provides a ripening controlling function, i.e., when the nucleic acid decomposition product is added before the conclusion of the first ripening (physical ripening), the growth of silver halide grains is controlled to result in the formation of finely divided particles or a change in crystal habit, which necessarily leads to a large change in photographic characteristics. Moreover, a nucleic acid decomposition product added before the conclusion of the first ripening remains in the emulsion after water washing, and influences the formation and growth of sensitive nuclei, causing a deterioration in sensitivity during the second ripening (chemical ripening).